(1) Field of the Invention
The present invention relates to a change speed control structure, particularly for use in an agricultural tractor, for controlling a multispeed transmission by manipulating a shift lever to operate hydraulic actuators.
(2) Description of the Prior Art
A known change speed control structure of this type is disclosed, for example, in U.S. Pat. No. 4,085,833. In this prior structure, a shift lever is guided by a straight guide groove defining a plurality of change speed positions from neutral to top speed arranged in series. This straight guide groove has the advantage of facilitating a change speed operations since the shift lever is movable linearly along the guide groove, namely in one action. However, there is the danger of a sudden starting, acceleration or deceleration resulting from an operation of the shift lever beyond a desired change speed position.
Proposals have been made in U.S. Pat. Nos. 2,917,941 and 4,216,680 to eliminated the above drawback by providing a stepwise guide groove crooked to define respective change speed positions of the shift lever. Such a guide groove does not include a straight section including a plurality of change speed positions, and therefore has the advantage of allowing reliable and safe change speed operations without a chance of the shift lever being switched beyond a desired change speed position. However, it has the disadvantage of very poor operability in that the shift lever must be flexed for movement from one change speed position to another.
Further, the same U.S. Pat. Nos. 4,085,833, 2,917,941 and 4,216,680 disclose link type relay assemblies for transmitting the movements of the shift lever to a hydraulic actuator control valve, and the Japanese utility model application laid open (Kokai) under No. 55-4974 discloses a wire type relay assembly which has a simpler construction and a greater adaptability than the link type.
This wire type relay assembly employs two wires for operatively connecting the shift lever to a change speed rotary valve to allow for a compact and bending interlocking system. The wires are wound on a small valve-side pulley and a large lever-side pulley so that an angle of rotation at the lever side is transmitted as enlarged to the valve side.
The valve-side pulley is rotatable by means of the two wires, in the clockwise direction by one of the wires and in the counterclockwise direction by the other, and therefore has an effective operating angle in the order of 120 degree with respect to the rotary valve. This construction results in the disadvantage that a large number of control ports cannot be provided without enlarging the spool diameter of the rotary valve.